The present invention relates to a method for recirculating the heavier solution in liquid--liquid extraction in between the solution separation part and mixing unit in a process where two unmixed solutions are first mixed and then the solutions are separated. The invention also relates to a collecting channel whereby the circulation of the heavier solution is realized. By means of the method and apparatus of the invention, the aqueous solution is gathered evenly also from a wide separation part, so that it does not disturb the proceeding of the extraction solutions flowing in the separation part. Another object of the invention is, by means of controlled circulation, to intensify the bottom flow in the longitudinal direction of the separation part and thus even out the vertical flows in this space.
When separating metals from each other, one of the employed separation methods is liquid--liquid extraction, where the heavier solution generally is an aqueous solution, and the lighter solution is an organic solution, such as kerosene, to which some suitable extraction chemical is dissolved. In large extraction plants, such as copper extraction plants, flow control is particularly important, even to an extent where it may prove to be a restricting factor as for the size of the plant in question.
Internal recirculation of solutions from the separation parts of single extraction steps back to the mixing unit of the same extraction step is very common, because the recirculation is part of the improving process of the solution-solution contact in the mixing tank. An equal quantity of both solutions is needed in the mixing tanks in order to achieve a desired solution contact, drop size and contact area. A desired solution contact indicates as to which solution is present in drops and which as a continuous solution. The mutual quantity of the solutions also affects the nominal viscosity of the dispersion. When the external supply of one of the solutions into the mixing tank is not sufficient, said solution is obtained into the circulation from the separation part. This situation concerns for example all solution contact steps of copper extraction, but particularly the washing step and the copper re-extraction step. For example in the washing of the extraction solution, the extraction solution flow can be 1,000-2,000 m.sup.3 /h, when the external basic solution feed is only of the order 10-50 m.sup.3 /h, i.e. in this case the internal recirculation need of the aqueous solution is nearly equal to the feed.
According to conventional practice, the internal solution circulation is obtained from chutes located at the discharge end of the separation part, to which chutes the purified solution is gathered as overflow. Especially when recirculation is large and the external solution supply small, it is difficult to make changes in the solution circulation quantities. It is particularly difficult to drive up a plant. There is no more solution to be used in recirculation than what is in the collecting chute and what is obtained as overflow. When for instance the circulation volume of the aqueous solution is increased, water runs up in the chute, and the desired circulation rate is obtained only gradually, as the external solution supply brings in more aqueous solution. This is due to the fact that an increase in the water supply increases the share of aqueous solutions in the mixing unit, and therefore respectively displaces organic solution. Thus the aqueous solution conducted into the mixing unit does not re-enter the circulation of the separation part, but the supply of aqueous solution is stopped for a long time. This leads to serious trouble in a process situation where it would be important to wash the extraction solution of difficult impurities; in copper extraction, these are, among others, chlorides, manganese and iron.